Organic or carbon semiconductor devices are promising for both nanoelectronic and macroelectronic applications. One of the major challenges to achieve high performance of these devices lies on understanding and improving the metal-organic (M/O) interface. In this paper, we present evidence and demonstration of Fermi-level depinning at the M/O interface by inserting an ultrathin interfacial Si3 N4 insulator in between. The M/O contact behavior is successfully tuned from rectifying to quasi-Ohmic and to tunneling by varying the Si3 N4 thickness within 0-6 nm. Detailed physical mechanisms of Fermi-level pinning/depinning responsible for the M/O contact behavior are clarified based on a lumped-dipole model and a simple depinning model. This work sheds light on the fundamental understanding of the M/O interface properties and also proves a practical engineering method of achieving low-resistance quasi-Ohmic contacts for organic electronic devices. © 2010 The American Physical Society.